The level of hardness in water can have a deleterious effect in many systems. For example, when hard water alone, or in conjunction with cleaning compositions, contacts a surface, it can cause precipitation of hard water scale on the contacted surface. In general, hard water refers to water having a total level of calcium and magnesium ions in excess of about 100 ppm expressed in units of ppm calcium carbonate. Often, the molar ratio of calcium to magnesium in hard water is about 2:1 or about 3:1. Although most locations have hard water, water hardness tends to vary from one location to another.
Water hardness has been addressed in a number of ways. One method currently used to soften water is via ion exchange, e.g., by adding sodium to the water to exchange the calcium and magnesium ions in the water with sodium associated with a resin bed in a water softening unit. The calcium and magnesium adhere to a resin in the softener. When the resin becomes saturated it is necessary to regenerate it using large amounts of sodium chloride dissolved in water. The sodium displaces the calcium and magnesium, which is flushed out in a briny solution along with the chloride from the added sodium chloride. When water softeners regenerate they produce a waste stream that contains significant amounts of chloride, creating a burden on the system, e.g., sewer system, in which they are disposed of, including a multitude of downstream water re-use applications like potable water usages and agriculture.
Hard water is also known to reduce the efficacy of detergents. One method for counteracting this includes adding chelating agents or sequestrants into detersive compositions that are intended to be mixed with hard water in an amount sufficient to handle the hardness. However, in many instances the water hardness exceeds the chelating capacity of the composition. As a result, free calcium ions may be available to attack active components of the composition, to cause corrosion or precipitation, or to cause other deleterious effects, such as poor cleaning effectiveness or lime scale build up.